A simple direct method for measuring the interference pattern of a light‐to‐recorded hologram phase shift in photorefractive crystals is reported. The method is used for studying the recording in Bi12SiO20 under an externally applied electric field. The results are then compared with theory and used for computing the density of photoelectron traps.

Phase‐shift measurement in photorefractive holographic recording

https://www.iept.tu-clausthal.de/fileadmin/homes/agkip/reprints/p05.pdf

Self-stabilized holographic recording in LiNbO3:Fe crystals

Holograms are thermally fixed in photorefractive lithium niobate crystals, i.e., they are recorded at temperatures between 130 and 180 °C. The setup is actively stabilized during recording against movements or vibrations of the interference pattern which especially occur during long-period writing at enhanced temperatures. Two different techniques are investigated: (1) Interference of the recording beams using one crystal surface as a beamsplitter yields a signal for stabilization. (2) Alternatively, one of the beams is periodically phase modulated and the beam-coupling signal is used for stabilization. Reproducible refractive index changes of thermally fixed holograms up to 7.5×10−4 are obtained with both stabilization techniques. However, the second method is advantageous for multiplexing experiments, because no readjustment of the beam-coupling stabilization system is required if the angles of the recording beams are changed.

Stabilized recording and thermal fixing of holograms in photorefractive lithium niobate crystals

Linear adaptive interferometers via diffusion recording in cubic photorefractive crystals

Applying external electric fields of as much as 65 kV/mm reveals significant enhancement of refractive-index changes Δns and of photorefractive sensitivity S. Samples with different iron concentrations and oxidation–reduction states were investigated. Refractive-index changes of up to Δns=11.5×10-4 and sensitivities as high as S=40 cm/J were measured (ordinary light polarization; light wavelength, λ=488 nm). The refractive-index changes are space-charge limited, but the photorefractive sensitivity S exhibits for large externally applied electric fields a linear increase up to the highest field values used. No nonlinear enhancement of the charge transport was observed. However, the excellent high-field material performance and the record values for Δns and S obtained improve the applicability of LiNbO3 crystals.

Improvements of sensitivity and refractive-index changes in photorefractive iron-doped lithium niobate crystals by application of extremely large external electric fields

We report the real-time direct interference-term measurement for a two-wave-mixing experiment in photorefractive crystals. Knowledge of the interference term may provide information concerning diffraction efficiency, interference pattern-to-recorded hologram phase shift, and optical activity and anisotropic diffraction properties of these materials. This method comprises phase modulation of one of the interfering beams and synchronous detection of the first and second harmonics in the resulting output irradiance modulation. Simultaneous detection of both harmonics enables the measurement to be made even in strongly perturbed conditions, since one harmonic is used for measuring and the other is used for operating an active stabilization system. Experimental results for Bi12TiO20 are reported.

Interference-term real-time measurement for self-stabilized two-wave mixing in photorefractive crystals.

We report the measurement of diffusion transport length LD, quantum efficiency φ for photoelectron generation, and the Debye screening length and associated trap density NA in an undoped Bi12SiO20 (BSO) photorefractive crystal. The method is based on the optical erasing of holographically recorded gratings in the crystal. The measurement of the diffraction efficiency evolution is considerably facilitated profitting from polarization properties of anisotropic diffraction in BSO‐type crystals in a self‐diffraction experiment. Data processing is carried out using simple linear regression techniques for experimental conditions in two limiting situations: far from and well into trap saturation conditions. We prove that optical erasure closely verifies the decay law predicted by theory even if a highly contrasted pattern of light is used for recording. We measured NA=6.3×1016 cm−3, agreeing in order of magnitude, with already reported values, but found out LD=0.1 μm, which is too much lower than data in literat...

Paulo Acioly M. dos Santos

Papers

Interference-term real-time measurement for self-stabilized two-wave mixing in photorefractive crystals.

Transport length, quantum efficiency, and trap density measurement in Bi12SiO20